Tensioning device for use particularly with vehicle safety belts

Abstract

A tensioning device which also provides for damped return movement, such as for use in tensioning a vehicle safety belt comprises a casing, a main piston movable within the casing by high pressure gases issuing from a pyrotechnic charge, a rod firmly attached to the piston and carrying an attachment buckle for the belt, and a piston jamming mechanism for preventing movement of the piston in one direction but allowing damped movement thereof in the opposite direction. The jamming mechanism is actuated in response to displacement of an auxiliary piston included in the main piston structure away from its initial position where it is held by a frangible element designed to break when the pressure of the combustion gases exceeds a particular level. The main piston preferably works in an outer piston which is slidable in the casing and also provided with a jamming mechanism whereby two phases of damping are provided for.

I claim: 
     
       1. A tensioning device for use in tensioning an element, such as a vehicle safety belt, comprising a casing, a cylinder having front and rear ends defined within said casing, a first piston axially movable within said cylinder, a rod firmly fixed to said first piston and having means for attaching said element to be tensioned, an expansion chamber defined between said first piston and said front end of said cylinder, means for supplying gas at high pressure to said expansion chamber to move said first piston in a first direction along said cylinder and exert a tensioning force on the element attached to said rod, an auxiliary piston included in the first piston structure, a frangible element for holding said auxiliary piston in an initial position and adapted to fracture when the pressure in said expansion chamber reaches a predetermined level, permanently open passage means communicating the pressure in said expansion chamber to said auxiliary piston, and piston stopping means actuated in response to displacement of said auxiliary piston away from said initial position following fracture of said frangible element, to prevent movement of said first piston relative to said cylinder in the said first axial direction, but allow movement of said first piston relative to said cylinder in the opposite direction. 
     
     
       2. A device as claimed in claim 1 wherein said first piston has a conical bearing surface on said piston stopping means comprises a jamming mechanism including a support surface, a plurality of elements positioned between said bearing surface and the inner surface of said cylinder and in contact with said support surface, and elastic retaining means for holding said elements in a position which permits free axial movement of said first piston relative to said cylinder, said support surface being movable by said displacement of said auxiliary piston to jam said elements between said bearing surface and said inner surface of the cylinder. 
     
     
       3. A device as claimed in claim 1 wherein said auxiliary piston is located axially within said first piston, said auxiliary piston includes two axially adjacent cylindrical parts defining a radial shoulder, and permanently open orifice means communicates the pressure prevailing in said expansion chamber to said shoulder. 
     
     
       4. A device as claimed in claim 1 wherein said auxiliary piston is annular and located at the periphery of said first piston, and permanently open orifice means communicates the pressure prevailing in said expansion chamber to said auxiliary piston. 
     
     
       5. A device as claimed in claim 1 wherein said frangible element is an integral part of said auxiliary piston, and said auxiliary piston is divided into two portions when said part fractures, with one of said portions being movable to actuate said piston stopping means. 
     
     
       6. A device as claimed in claim 1 wherein said frangible element is separate from said auxiliary piston, and the whole of said auxiliary piston displaces upon fracture of said element to actuate said piston stopping means. 
     
     
       7. A device as claimed in claim 1 wherein said means for supplying gas to said expansion chamber comprises an annular pyrotechnic charge located inside said casing, and means for igniting said charge. 
     
     
       8. A device as claimed in claim 7 wherein said pyrotechnic charge has a surface area of combustion which increases during combustion of said charge. 
     
     
       9. A device as claimed in claim 8 wherein said igniting means comprises a gas-free powder. 
     
     
       10. A device as claimed in claim 1 wherein said cylinder is constituted by an outer piston which is axially movable within said casing, the outer surface of said outer piston partially defines a first damping chamber, and said outer piston has means for preventing movement of said outer piston relative to said casing in one axial direction of movement of said outer piston relative to the casing, which said direction corresponds to return movement of said outer piston to an initial position relative to said casing.